Why general population want to have a 6-pack abs?
You don't need a clear 6-pack to have a strong rectus abdominis muscle.... Also when i think about it, it's quite useless, unless you learn martial art and need a strong muscle to protect your stomach. Then unless you like swimming or walk around shirtless, it's not visible most of the time anyway

My biggest guess is because it's one of the milestone for people to achieve (much like dunk in basketball)

Not many people want bodybuilder-like sixpacks (hypertrophy, you're trying too hard), but most everyone wants to look lean and somewhat muscular. The best way to this is to become fit and in the process remain within a healthy weight range, because a sixpack will show on someone who's not overweight regardless of whether they do crunches every night or not.

Aesthetically guys do all this to impress girls, which is half the reason we breathe.